The present invention relates to a method of producing a uniform silica glass block, and more particularly to a method of producing a uniform silica glass block having no voids and cords, and also having transmittance of light beam ranging from infrared to ultraviolet equal to or better than that of presently available optical synthetic silica glass. Further, it relates to a method of producing a uniform silica glass excellent not only in the above properties but also in heat resistance.
Silica glass blocks, particularly anhydrous silica glass blocks, are presently produced by the following methods:
(1) Melting method using natural rock crystals as raw materials; PA1 (2) Bernoulli method using SiCl.sub.4 as a raw material and converting it to SiO.sub.2 by an oxygen plasma flame; and PA1 (3) VAD method (soot method) using SiCl.sub.4 as a raw material and converting it to a porous body (preform) of SiO.sub.2 with an oxyhydrogen flame and sintering it to provide a silica glass block.
However, the silica glass produced by the above methods has various problems. Specifically, in the case of the method (1), the resulting silica glass has nonuniform portions such as voids, cords, etc., and since natural rock crystals are used, the silica glass does not have sufficiently high purity. In the case of the method (2), nonuniform portions such as voids and cords are likely to be generated, and chlorine tends to remain in the resulting glass block in an amount of 200 ppm or so. Further, in the case of the method (3), it is difficult to provide a large silica glass block.
In general, once voids or cords are introduced into the glass, it is extremely difficult to remove them, so that the resulting glass cannot be used as optical silica glass which is required to have extremely high uniformness.
In view of the above problems, the inventors previously filed a patent application for a method of removing voids and cords from silica glass by a high-temperature, high-pressure treatment (Japanese Patent Application No. 62-24702). In this method, the voids and cords generated in the silica glass in its production step are removed by a subsequent high-temperature, high-pressure treatment. However, this treatment does not sufficiently remove the voids and the cords, and this treatment undesirably increases the production costs of silica glass blocks. Therefore, it has been desired to find out conditions under which any voids or cords are prevented from being generated in silica glass blocks, from the viewpoint of production costs.
In general, to achieve high uniformness, it is desired to melt glass to a low viscosity. In this respect, since usual optical glass, which is made of a plurality of components, has a lower softening temperature than the silica glass, it can have a viscosity of 10.sup.5 poise or less by heating. Thus, by stirring at such a viscosity, high uniformness can be achieved.
However, since the silica glass has a high softening temperature, it has a high viscosity even when heated up to 2000.degree. C., making it difficult to make it uniform by stirring. Further, even if it is tried to reduce its viscosity at which its stirring is possible, by heating it to as high a temperature as 2000.degree. C. or more, it would practically be difficult to find refractory materials usable at such a high temperature. In addition, at such a high temperature, silica is extremely vaporized under normal pressure, causing the problem of extreme material losses. Accordingly, the heating of the silica glass to an extremely high temperature is not a practical method as a means of providing uniform silica glass.
In view of the above, to obtain optically uniform silica glass, large ingots are presently produced and proper sizes of glass blocks are cut out from them, and uniform portions are selected from the cut portions, and ground and polished to final shapes.
However, for the purpose of obtaining high purity, uniform silica glass block, the above method is disadvantageous in that the yield of final products is extremely low, making the production costs extremely high. In addition, this method fails to provide silica glass blocks of arbitrarily desired shapes without additional working.